Elucidate Phonon Dynamic of Grain Boundaries in Perovskite Solid-State Electrolytes by Electron Microscopy

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All-solid-state batteries (ASSBs) are set to revolutionize the future of lithium-ion batteries (LIB) [1,2], with solid electrolytes being essential in defining their performance. Grain boundaries (GB) pose significant challenges by impeding ion transport and thermal conduction, thus representing a major obstacle in enhancing the properties of solid electrolytes. As a result, the study of the atomic structure and thermal transport of GBs has become a focal point in materials science research. Recently, there has been growing interest in compositionally complex ceramics, including high-entropy variants [3,4], for the development of high-performance solid electrolytes. A notable example is (Li0.375Sr0.4375)(Ta0.375Nb0.375Zr0.125Hf0.125)O3 (LSTNZH), which has demonstrated potential in boosting GB ionic conductivity [5], with further improvements achievable through quenching. Besides ionic conductivity, the thermal properties of solid-state electrolytes are vital for enhancing safety and preventing thermal degradation. By employing advanced scanning transmission electron microscopy (STEM) techniques, particularly vibrational electron energy-loss spectroscopy (EELS) with atomic spatial resolution and few-meV energy resolution, we conduct a comprehensive investigation into the atomic structure and phonon dynamics of GBs in LSTNZH subjected to air quenching (AQ) and furnace cooling (FC). Our findings shed light on the phonon transport mechanisms at GBs in LSTNZH, offering valuable insights for the advancement of next-generation solid-state electrolytes. The LSTNZH ceramics were synthesized using a conventional solid-state reaction method. One set of samples was quenched in air at a rapid cooling rate, while the other set was allowed to cool slowly in the furnace after sintering. The ionic conductivity of each sample group was measured using electrochemical impedance spectroscopy (EIS). Fig. 1A presents Nyquist plots for the furnace-cooled LSTZ (gray curve) as a baseline, alongside LSTNZH (red curve) and air-quenched LSTNZH (cyan curve), suggesting that air-quenched LSTNZH has the best ionic conductivity, especially at GBs. Thermal conductivity measurements, shown in Fig. 1B, indicate that furnace-cooled LSTNZH exhibits higher thermal conductivity than its air-quenched counterpart. Notably, thermal conductivity increases with temperature, suggesting that heat conduction is predominantly influenced by interfacial processes, particularly at GBs. We first examined the grain size of both sample groups using electron backscatter diffraction (EBSD) to eliminate the effect of GB quantity. As shown in Table 1, the average grain size and standard deviation are similar within an acceptable tolerance, indicating that the GB quantity and length are comparable between the two sample groups. The microstructures of the GBs in our two perovskite solid electrolytes were analyzed. Fig. 1C displays an atomic resolution high-angle annular dark field (HAADF) STEM image of a randomly selected GB from the air-quenched LSTNZH, with atoms of nine A-site centered unit cells labeled to illustrate the atomic arrangements. A similar image for a GB of furnace-cooled LSTNZH is shown in Fig. 1D. Since the crystal lattices extend continuously from the bulk to the GB, there is no additional energy barrier for phonon transport due to changes in lattice structure. Using atomic-resolved energy dispersive X-ray spectroscopy (EDS), the atomic percentage of B-sites near the GBs is shown in Fig. 1E (air-quenched LSTNZH) and Fig. 1F (furnace-cooled LSTNZH). The line profile clearly indicates elemental segregation at the GB of air-quenched LSTNZH, whereas a uniform elemental distribution is observed at the GB of furnace-cooled LSTNZH, suggesting different phonon transport mechanisms across the GBs in these two groups of samples. To understand the impact of GBs on thermal conductivity, we will sequentially use atomic resolution dark-field vibrational EELS to map phonon populations near the GBs. By comparing the differences in phonon populations between the GBs of the two sample groups and integrating other state-of-the-art techniques, we aim to gain a deeper understanding of how material processing affects GB thermal resistance. From a broader perspective, this research lays the groundwork for designing next-generation solid-state electrolytes with improved ionic conductivity [6]. A AC impedance spectra of air-quenched (cyan dots), furnace-cooled (red dots) LSTNZH (x = 9/16) and furnace-cooled LSTZ (y = 0.75) (gray dots) samples measured at room temperature. Curves are fitted and displayed over the spectra accordingly. B Thermal conductivity of air-quenched (cyan dots) and furnace-cooled (red dots) LSTNZH (x = 9/16) measured in 60 K to 400 K range. The 10% error bars primarily stem from uncertainties in the sample geometry. Atomic-resolution HAADF-STEM images and B-sites atomic percentage line profile of C, E air-quenched LSTNZH and D, F furnace-cooled LSTNZH grain boundaries. HAADF images are overlapped with related atomic models with the illustration of each site below. Atomic percentage line profile was measured by EDS. Scale bars are 2 nm. Grain size measurements Grain size measurements

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